The installation of cryogenic turbine expanders in existing LNG plants presents a very effective upgrade with an advanced but field proven technology requiring relatively low investment costs and only short installation time. All LNG plants commissioned before 1996 are currently operating with an ineffective expansion valve. By replacing the existing Joule-Thomson valve a.k.a. J-T valve with a cryogenic turbine to expand the condensed natural gas from high pressure to low pressure, the thermodynamic efficiency of the existing refrigeration process is substantially improved, providing an increase of the total LNG output between about 5 to 7%. To attain this increase in efficiency, no substantial investments are necessary.